The present invention relates to integrated circuits, and more particularly, to an integrated bandgap reference circuit operative to generate an output voltage that is adapted not to vary with temperature.
Bandgap reference voltage generators (alternatively referred to as bandgap reference circuits) are used in a wide variety of electronic circuits, such as wireless communications devices, memory devices, voltage regulators, etc. A bandgap reference circuit often supplies an output voltage that is relatively immune to changes in input voltage or temperature.
A bandgap reference circuit is typically adapted to use the temperature coefficients associated with physical properties of the semiconductor devices disposed therein to generate a nearly temperature-independent reference voltage. A bandgap reference circuit operates on the principle of compensating the negative temperature coefficient of VBE—which is the base-emitter voltage of a bipolar transistor—with the positive temperature coefficient of the thermal voltage VT. In its most basic form, the VBE voltage is added to a scaled VT voltage using a temperature-independent scale factor K to supply the reference voltage Vref, as shown below:Vref=VBE+K*VT  (1)
Because voltage signals VBE and VT exhibit opposite-polarity temperature drifts, parameter K may be selected such that voltage Vref is nearly independent. As is known to those skilled in the art, thermal voltage VT is equal to kT/q, where, where k is Boltzmann's constant, T is the absolute temperature in degrees Kelvin, and q is the electron charge.
In addition to being temperature independent, a bandgap reference circuit is ideally also adapted to supply a substantially stable and unchanging output reference voltage despite variations in the input voltage levels received by or the capacitive loading applied to the bandgap circuit. Accordingly, an ideal bandgap reference circuit output is also immune to ripples or noise that is typically present in the power source supplying voltage to the bandgap reference circuit. However, most bandgap reference circuits exhibit non-ideal characteristics. One measure of the ability of a bandgap reference circuit to suppress or reject such supply ripple or noise voltages is referred to as the power supply ripple rejection (PSRR).
The growth in demand for battery-operated portable electronic devices, such as wireless communications devices and personal digital assistance devices, has brought to the fore need to develop low voltage, low power systems. For instance, many portable wireless systems are being designed to operate using batteries that supply, for example, 1.3 volts. Designing a bandgap reference circuit adapted to operate at such low voltages poses a challenging task.
In a publication entitled “A Sub-1-V ppm/° C. CMOS Bandgap Voltage Reference Without Requiring Low Threshold Voltage Device”, IEEE Journal of Solid State Circuits, Vol. 37, No. 4, April 2002, pp. 526-530, authors Leung et al. propose a sub-1V bandgap reference voltage formed using a standard CMOS process and that dispenses with the need for low threshold voltage devices (such as those shown in FIGS. 1A and 1B of Leung et al.). FIG. 1 shows a transistor schematic diagram of the sub-1V bandgap reference circuit 10 by Leung et al.
The sub-1V bandgap reference circuit 10 includes a single loop and a single operational amplifier 12 that receives input voltages from node N1 and N2. Current I is generated by the closed-loop circuitry formed by operational amplifier 12, transistors Q1, Q2, and resistors R1, R2A1, R2B1, R2A2 and R2B2. Current I has the following magnitude:I=VEB/R2+VT*ln N/R1  (2)where N is the ratio of the emitter areas of transistors Q1 and Q2, VT is the thermal voltage and where:R2=R2A1+R2A2=R2B1+R2B2  (3)
Transistors M1, M2 and M2 form a current mirror. Therefore current I flowing through resistor R3 is equal to the current that also flows through transistor M1 or M2. The reference voltage Vref generated by the bandgap reference circuit 10 is as follows:Vref=(R3/R2)*[VEB2+(R2R*ln N/R1)*VT  (4)
Parameter N is selected such that voltage Vref is nearly temperature-independent. As is seen from the above, bandgap reference circuit 10 includes single closed-loop circuitry that causes the same current I to flow through output transistor M3. Therefore, if another output stage (not shown—but similar to that formed by transistor M3 and resistor R3) is disposed between supply voltage Vdd and the ground terminal, it will generate an output voltage with the same nearly zero temperature coefficient as that of Vref.
There may be instances where at least two reference voltages each with a different temperature coefficient may be required. For example, to compensate for a positive temperature drift of a voltage-controlled oscillator, it may be desired to generate an output reference voltage that has a negative, non-zero temperature coefficient. Two different bandgap reference circuits 10 (i.e., with different physical parameters) would be required to generate two reference voltage that have different temperature coefficients., thereby increasing cost.
There continues to be a need for a bandgap reference circuit that is scalable and is thus adapted to generate multiple output reference voltages with each output reference voltage having a different temperature coefficient.